Counter surveillance professionals are obviously interested in locating concealed surveillance devices. Often these concealed surveillance devices are coupled to communication or power transmission lines so that they can monitor any transmissions on the line or use the lines to complete their own covert transmissions. The coupling of the surveillance device to the transmission lines usually creates an impedance anomaly on the line that affects the transmission of signals along the line under certain circumstances. Therefore, counter surveillance professionals need a way to locate impedance anomalies on transmission lines caused by surveillance devices coupled to the lines such that the surveillance devices can be removed.
Telecommunications and wiring inspection applications in the airline and automobile industries require examining wires and structures for the presence of harmful corrosive junctions and/or proper wire interconnections. Prior art methods for inspecting such systems are not able to both detect and classify impedance anomalies as either corrosive junctions or semiconductor device based connections. Therefore, an improved way of inspecting wired networks is needed.
Time domain reflectometry (TDR) has been used by telecommunications providers as a testing device to locate impedance anomalies on their existing telephone transmission lines. These transmission lines typically contain one or more impedance anomalies due to connections to the lines by devices such as load coils and bridged taps. However, impedance anomalies may occur due to installation errors, faulty lines, or surreptitious connections. These anomalies result in a loss of transmission efficiency and performance because a portion of the energy propagating along the transmission line is reflected back in the direction of the transmitting source. These reflections can cause a significant reduction in signal amplitude when the reflected signal is combined with the original signal, thereby, disrupting any normal transmissions on the line. Time domain reflectometry is typically used to locate and quantify the significance of these anomalies.
Time domain reflectometry involves transmitting a test signal down the transmission line and measuring a time delay between the transmitting of the test signal and the reception of a reflection. If the velocity of propagation for the line is known, the distance to the reflecting impedance anomaly can be determined. Time domain reflectometry can provide some information about the type of impedance anomaly such as is the impedance anomaly more inductive or capacitive. It can also provide accurate range information to the anomaly. Unfortunately, time domain reflectometry cannot provide information about whether or not the anomaly is due to an electronic surveillance device or a mechanical connection or installation imperfection. Thus, since transmission lines typically have several impedance anomalies, a counter surveillance professional interested in locating concealed surveillance devices will be required to manually inspect each identified impedance anomaly to determine if the anomaly is the result of a concealed surveillance device.
Frequency domain reflectometry (FDR) can provide impedance information similar to time domain reflectometry, however frequency domain reflectometry uses a frequency swept waveform to detect any impedance anomalies on a transmission line. As the frequency of the waveform is swept through a range of frequencies, the transmission line's response to the swept waveform is monitored. The amplitude of the measured signal response will exhibit a variation with frequency that is a composite of the fluctuations in impedance due to any reflection points along the line under test. By examining these variations in the line's response to the swept frequency waveform, the location of the impedance anomalies can be determined. Like a time domain reflectometry process, an frequency domain reflectometry process can also provide information regarding the range and nature of the impedance anomaly, but it cannot predict whether or not the anomaly was caused by an electronic device or by some other type of discontinuity.
Therefore, in view of the above discussed deficiencies in the prior art, what is needed is an improved ability to detect impedance anomalies on a transmission line that correspond to electronic devices coupled to the transmission line.